I used share my DSL with my USB modem too before I got my router. Like wedor said, set your main (or fastest) computer as you "server" using ICS, then connect the other computer "clients" to your NIC (w/crossover cable) or a hub.
http://www.dslreports.com
http://www.practicallynetworked.com/
Great places for info on setting up a small network.